Crystallographic Studies of Mayenite and Isostructural Compounds for Understanding Host Lattice and Guest Interactions: Experiment and Simulation

Author

Kathleen Cole Loughlin, University of Tennessee, KnoxvilleFollow

Date of Award

12-2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Claudia J. Rawn

Committee Members

David J. Keffer, Haixuan Xu, Michael R. Koehler

Abstract

Mayenite, Ca₁₂Al₁₄O₃₃, comprises a positively-charged framework of cages, [Ca₁₂Al₁₄O₃₂]²⁺, with 1 in 6 cages occupied by O^2-. Many potential applications depend caged anions’ ability to move through the material and/or be replaced by other anions. Sr and Ga can replace the divalent and trivalent cations, respectively, forming the isostructural compounds Sr₁₂Al₁₄O₃₃ and Ca₁₂Ga₁₄O₃₃with larger lattice parameters and, necessarily, changes in the mobility of caged ions.

As polymeric steric entrapment had previously achieved direct or near-direct synthesis of Ca₁₂Al₁₄O₃₃ and Ca₁₂Ga₁₄O₃₃, it was used to synthesize an isostructural strontium aluminate compound. The majority phase (80.3(5) wt%) was Sr₁₂Al₁₄O₃₂Cl₂ with SrCl₂·6H₂O and Al(NO₃)₃ as precursors, but there was no evidence of a mayenite-type phase with Sr(NO₃)₂ and Al(NO₃)₃ as precursors, suggesting that the larger anion was necessary to template the cage and form the phase. High-temperature X-ray diffraction showed a more complex synthesis route for Sr₁₂Al₁₄O₃₂Cl₂ using polymeric steric entrapment than for either Ca₁₂Al₁₄O₃₃ or Ca₁₂Ga₁₄O₃₃.

Classical molecular dynamics using core-shell Buckingham potentials was used to compare oxygen mobility in 4x4x4 unit cells of Ca₁₂Al₁₄O₃₃, Ca₁₂Ga₁₄O₃₃, and Sr₁₂Al₁₄O₃₃ at 77, 300, 700, and 1673 K. At 700 and 1673 K, simulations saw initial caged oxygens in Ca₁₂Al₁₄O₃₃ and Ca₁₂Ga₁₄O₃₃ move through the framework, combining exchange with framework oxygens and vehicular motion through cage windows, but only at 1673 K was there evidence of limited mobility of the caged oxygens in Sr₁₂Al₁₄O₃₃. Framework exchange was the main driver for Ca₁₂Ga₁₄O₃₃, which had the highest overall oxygen mobility of the three.

CHGNet machine learning interatomic potentials were used with classical molecular dynamics to simulate 3x3x3 unit cells of [Ca₁₂Ga₁₄O₃₂]²⁺ at 700 K with O^2-, 2 O^-, 2 Cl^-, and 2 F^- as cage occupants. Both oxygen species sat off-center in the cage and underwent both exchange and vehicular motion; both halides remained in the cage center and did not diffuse. Lattice parameters increased with larger occupants. Results from core-shell Buckingham and CHGNet potentials were qualitatively similar; the Buckingham potentials were much faster, but the CHGNet potentials allowed greater flexibility with atomic species. Further work is required to determine which better models the system.

Recommended Citation

Loughlin, Kathleen Cole, "Crystallographic Studies of Mayenite and Isostructural Compounds for Understanding Host Lattice and Guest Interactions: Experiment and Simulation. " PhD diss., University of Tennessee, 2025.
https://trace.tennessee.edu/utk_graddiss/13614